In wireless communication, a generated transmission signal is amplified to a predetermined power by an amplifier to be transmitted. As for the amplifier, the power efficiency is higher as it is operated at an amplification factor in a region closer to a saturation level. However, when a signal having a high peak-to-average power ratio (PAPR) such as an orthogonal frequency division multiplexing (OFDM) signal is amplified, a large backoff is taken, thereby lowering the power efficiency of the amplifier.
As a method of improving the power efficiency of the amplifier, for example, a method of clipping a predetermined amplitude and higher in a transmission signal, and cutting out-band components caused thereby of the transmission signal with a band limiting filter has been known. In this method, out of frequency components of the clipped transmission signal, out-band frequency components of the transmission signal is extracted by a high-pass filter, and the extracted frequency components are subtracted from the frequency components of the clipped transmission signal. Thus, frequency components that suppresses predetermined amplitude and higher in the transmission signal remain, and a transmission signal in which the predetermined amplitude and higher are suppressed is input to the amplifier. Prior art example is disclosed in Japanese Laid-open Patent Publication No. 2008-141415.
The frequency components to suppress predetermined amplitude and higher in a transmission signal are noise for the transmission signal. Therefore, if frequency components to suppress peaks of the amplitude in the transmission signal are included in a band of the transmission signal, the quality of the transmission signal such as an error vector magnitude (EVM) is degraded.